The aerospace giant Airbus hopes to put a hydrogen-powered commercial airliner in the sky that will release zero carbon dioxide emissions in the atmosphere. But not until 2035.
While 15 years might seem like a long time for research and development given the urgent need to reduce carbon emissions under the Paris climate agreement, processing and storing "clean hydrogen" requires solving an array of complex technical challenges. Three early design concepts the company is studying would run off of hydrogen and oxygen fuel and have no carbon exhaust. But that doesn't mean they won't affect the climate at all.
"I will let you in on a little secret, they are not zero emission," Amanda Simpson, vice president for research and technology for Airbus Americas, said.
Burning hydrogen produces water, which comes out of the engines as a vapor that, especially at high altitudes, acts as a greenhouse gas.
Recent studies have shown that contrails—the white streaks of condensed water that follow jets across the sky—have a significant climate impact. Still, these hydrogen-powered designs could significantly limit the total warming that airlines cause by reducing or eliminating the carbon dioxide they emit. Airlines accounted for more than 2 percent of global CO2 emissions in 2018, with the total contribution of contrails and the various pollutants from commercial aviation driving about 5 percent of warming globally.
Up to this point, industry attempts at zero carbon flight have been smaller proof-of-concept designs, like short range electric planes that don't scale up practically for larger passenger flights.
Simpson said she thinks hydrogen power is going to be "as clean as we can get," so the development of a plane that runs on it is an important step in decarbonizing the aerospace industry.
Ilan Kroo, a professor of aeronautics and astronautics at Stanford University, said there's no guarantee that Airbus can meet their self-imposed deadline of 2035, because there are many uncertainties along the road to developing hydrogen-powered planes. No one knows what problems will arise or what breakthroughs may occur over the next 15 years.
"I think it's great that they're hoping to do that," he said.
Annie Petsonk, international counsel for the Earth Defense Fund, said she thinks it's crucial that Airbus is "giving engineers a goal and a commitment." For these technologies to progress, she said, companies and policymakers need to set targets, then solve the technological hurdles to meet them, rather than the other way around.
Creating Demand for Hydrogen-Powered Flight
Airbus has previously made public forays into zero carbon flight, beginning with a small electric aircraft unveiled in 2010. The company's main competitor, Boeing, tested a small hydrogen powered aircraft in 2008—the first manned flight of its kind—and test flew a hydrogen drone in 2012, but hasn't announced plans for hydrogen-powered passenger flights.
In fact, Boeing's top product developer, Michael Sinnett, recently expressed skepticism at the possibility of hydrogen-powered passenger flights in the near future. Sinnett said that the new hydrogen technologies would take a long time to develop and that government regulations might not be quick to accommodate them, once they arrived. He also said that to maintain aircraft safety standards, the industry still has a lot to learn about hydrogen.
The hydrogen on the market today, said Simpson, of Airbus Americas, is "brown," meaning that it is not a sustainably-produced energy source. Ninety-five percent of hydrogen produced in the United States comes from natural gas "reforming," an energy intensive process that converts methane into hydrogen and other gases. Currently, the vast majority of domestic natural gas comes from hydraulic fracturing.
But in creating a demand for clean hydrogen, Simpson said, Airbus anticipates that fuel suppliers will upgrade their production of the element by splitting water molecules into oxygen and hydrogen using renewable electricity.
"If demand is going to support them," Simpson said of hydrogen-powered airliners, "we think they will become economical and they will be very clean."
Airbus plans to have several hydrogen-powered planes flying by the mid-2030s, Simpson said, and from there the company will gradually grow its fleet.
Over the next five years, Airbus will do intensive research and development for its hydrogen concepts, Simpson said. They presented preliminary designs in an announcement last month: one for a passenger jet and another for a smaller capacity "turboprop "plane driven by a turbine combined with a propeller. Both these designs resemble conventional passenger aircraft, but Airbus has also conceived of a wide "blended-wing body," that would have a similar range and passenger count as the jet, but that looks more like a stealth bomber.
The larger designs would seat 200 passengers and have a range of 2,000 nautical miles, while the turboprop would seat 120 and have a range of 1,000 miles. They would burn hydrogen and oxygen and draw additional power from hydrogen-oxygen fuel cells—devices which, rather than burning the elements, combine them to produce electricity, like a battery that, instead of being recharged, is refilled with the elements that react to create electricity. The fuel cells would provide an extra boost through an electric engine.
These concepts are early, and Airbus may end up developing more than one of the three concept planes or coming up with an entirely new concept by the end of the five years. "One? Two? All three? Something different?" Simpson said.
The final design will depend on input from airlines and, in turn, passengers' interest in low-carbon flight, she said.
The five year research and development period will be followed by one to two years of "industrializing"—figuring out how they would design, build and upscale the aircraft. They would expect the first flight after four or five more years, and then would begin testing the aircraft to ensure it meets international aviation standards. Certifying the aircraft will inherently be more difficult, because of the novelty of its hydrogen-adapted components.
Finding Room for Fuel
Fuel storage is a major technical hurdle for hydrogen aircraft. Gaseous hydrogen fuel would weigh less than conventional fuels with the same amount of energy, but take up more space, Kroo said.
This would require aircraft designers to either find a way to safely and efficiently store liquid hydrogen, which is much more compact than the gaseous form, or find more room onboard the aircraft for fuel storage.
Whether aircraft will be able to store liquid hydrogen is a current debate, Kroo said. The fuel used in many rockets, liquid hydrogen would require heavy, high-pressure storage tanks kept at ultra-low temperatures. If designers can make these tanks light, cool and safe, they could pack a lot of energy into a small space.
Otherwise, they'll have to find more space in the aircraft for gaseous hydrogen. One advantage of the "blended-wing body" design, according to Kroo, is that it could create more internal space for fuel storage.
In addition to the water vapor that forms contrails, the hot turbines, like those in current aircraft, will fuse together nitrogen oxides from the nitrogen and oxygen in the air, but hydrogen turbines would produce considerably fewer than conventionally fueled ones, Kroo said. Nitrogen oxides create ozone. In the stratosphere the ozone layer blocks ultraviolet light that can burn skin and plants, but at high elevations in the troposphere, the layer of the atmosphere closest to the Earth, ozone and water vapor can act as greenhouse gases.
"In general, hydrogen [fueling an aircraft] at altitudes below 11 or 12 kilometers, has substantially lower greenhouse gases," than an aircraft that uses conventional jet fuel, Kroo said. To limit the climate impact of nitrogen oxides and vapor trails, he said, airlines should avoid pushing the cruising altitudes of their passenger jets higher than that, which would exacerbate the climate-warming effects of both.
Flying higher is generally appealing for airlines because the thinner air causes less drag and allows for more efficient flights. Some studies have shown that flying at lower altitudes can significantly reduce production of nitrogen oxides. Others have shown that altering flight routes to avoid forming contrails could reduce their warming impact by roughly 50 to 60 percent. He also noted that an electric-only hydrogen fuel cell-powered plane could avoid the water vapor problem entirely, by trapping the water formed in the process inside the plane.
As for shorter term sustainability measures, Airbus is looking at using synthetic fuels that can be produced by combining hydrogen and carbon dioxide captured from the air to replace conventional ones, namely, kerosene, which are refined from oil. These fuels can be used in current aircraft as an "intermediate step" before the hydrogen-fueled planes come on to the market.
"Synthetic fuels are indeed a way to decarbonize, but it's not zero emission," Simpson said. At present, the aviation industry depends on long-range, high-occupancy aircraft. These early hydrogen designs can't fill that role, and neither can shorter-range electric planes, Simpson said. "So synthetic fuels are going to have to be part of the solution to decarbonize aviation."
The switch to hydrogen, she said, will not happen overnight. More than 20,000 airliners are in service today, and that number is expected to jump to between 30,000 and 40,000 by 2030.
"There's gonna be kerosene...in use for a long, long time," she said. Still, she is confident that green hydrogen fuels will be available in the not too distant future and engines that run on it will eventually be developed.
"Hydrogen is coming," she said. "We're just going to be adding to the demand."